'Lost years' end for backyard supernova

As the first findings start to arrive from the Hubble Space Telescope since its repair last year, researchers are shedding new light on one of our nearest and most exciting supernova neighbours as they resume tracking its explosive history.

Supernovae form when a massive star explodes at the end of its life. Opportunities to view the event in a nearby galaxy are scarce, but in 1987 just such an explosion was observed in one of our nearest galaxies, the Large Magellanic Cloud. By a happy coincidence, Hubble was launched only 3 years later, and has been tracking the development of the remnant debris of supernova 1987A throughout its infancy. This is a rare and exciting opportunity, says Kevin France, an astrophysicist at the University of Colorado, Boulder, and lead author on the study.

"We've got this unprecedented picture. We caught it from pretty much day one and we have been watching it ever since--it's kind of in our cosmic backyard," he says.

But Hubble's tracking of Supernova 1987A stopped cold in August 2004, when a power failure occurred in the Space Telescope Imaging Spectrograph. This instrument collects observations of the wavelengths of light radiated by gases, creating a 'spectral fingerprint' of the elements present. The problem was not fixed until May 2009.

A team of researchers led by France was among the last to observe supernova 1987A through Hubble in 2004, and has now collected some of the first data since its repair. They published their results in the September 3 issue of Science.

Supernova 1987A is surrounded by a ring of debris that was thrown out by the star 20,000 years before it exploded into a supernova. The team's most recent observations of hydrogen radiation in the visible and ultraviolet spectra show that emissions from the supernova are brighter than those observed in 2004. The researchers suggest that the shock waves of gas sent out by the supernova are brightening the ring of debris round it (seevideo).

They also think that some of the shock waves are bouncing back off the debris. The team has used Hubble's imaging spectrograph to map the velocity and chemical composition of this moving gas.

"Although we always knew it was there, this is the first time we've actually been able to see this reverse shock [in supernova 1987A]", says Richard McCray, an astrophysicist at the University of Colorado and a co-author on the study.

Because both the visible and ultraviolet radiation that the team observed were coming from hydrogen atoms excited by the moving shock wave, both types of radiation were expected to have the same velocity. So the team was surprised to find that there was more ultraviolet radiation than expected, and that its velocity was higher than that of the visible radiation. The "enormous" discrepancy, says McCray, "was just staring us in the face".

To explain this finding, the team proposes a new mechanism in which the ring of debris itself emits radiation in the ultraviolet part of the spectrum, with this being scattered back towards the observer by the gas from the supernova.

"It's kind of like a moving mirror, and it's flashing this ultraviolet light to us," says Robert Kirshner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and a study co-author.

"It doesn't completely change ideas in the field," says Roger Chevalier, a theoretical astrophysicist at the University of Virginia, Charlottesville. But he adds: "This is a significant step in understanding this object. It is expected to get brighter over the years, so this provides a baseline from which we can build a more complete picture."

Such research is crucial to understanding the bigger picture of how supernovae affect their environment, says France. Supernovae have an important role in the evolution of galaxies, because they input a massive amount of energy into the environment, and they are the source of most heavy elements, such as iron, he says.

"With this research we can start to understand the chemical and physical impacts of the supernovae," France says. "And we're actually seeing this on human timescales, which is pretty cool, because most things in astronomy happen on enormous timescales."

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